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Acids and Bases - Conjugate Pairs

A second part is devoted to the subject of conjugation of acids and bases. The relationship between the acidic constant Ka, basic constant Kb, and the constant of autoionization of water, Kw will be discussed. The relationship is useful for weak acids and bases.

Skills to Develop

  • Give three definitions for acids.
  • Give three definitions for bases.
  • Explain conjugate Acid-Base pairs.
  • Give the conjugate base of an acid.
  • Give the conjugate acid of a base.

Acids and Bases

The primitive way to characterize a substance is by tasting. Acids taste sour, and bases taste bitter.

Acid Formula
Acetic \(\ce{CH3COOH}\)
Hydrochloric \(\ce{HCl}\)
Sulfuric \(\ce{H2SO4}\)
Nitric \(\ce{HNO3}\)
Carbonic \(\ce{H2CO3}\)
Base Formula
soaps Varies

At this point of your development, you already know some acids and bases. Are you familiar with the acids and bases listed here? If not, you probably will learn a lot here.

Acids and bases are also powerful concepts used to characterize substances. The concepts can and have been extended to much wider applications. For these reason, there are at least three definitions each of acid and base. These definitions are given at different times to expand the concepts of acids and bases. This is why we do not even define acids and bases in this section.

Let us take a look how the concepts of acid and base are expanded over time. Acids and bases are related, and the relationship is called conjugation. An acid has a conjugate base and vice versa.

Evolution of the Acid-Base Concept

In 1884, Arrhenius noticed that all acids have \(\ce{H+}\) ions and bases have \(\ce{OH-}\) ions. Thus, he considered all substances giving \(\ce{H+}\) and \(\ce{OH-}\) ions are acids and bases respectively.

Less than 40 years later, in 1923, Bronsted, Lowry and Lewis wanted to expand the concepts of acids and bases so that a wider area of chemistry can be understood using the same principle.

  Acid Base Neutraliztion
\(\mathrm{H^+ + OH^- \rightarrow H_2O}\)
\(\mathrm{HA + B \rightarrow A + HB}\)
Electrophile Nucleophile \(\mathrm{E + \textrm{:Nu} \rightarrow \textrm{E:Nu}}\)

Bronsted-Lowry's concept focused on the proton, and defined

  • Acids as proton donors,
  • Bases as proton acceptors,

whereas Lewis focused on the electrons and defined

  • Acids as electrophiles,
  • Bases as nucleophiles.

Lewis definitions are more general than Bronsted-Lowry's definitions; they are now widely accepted and practiced. The more general definitions for acids and bases allow us to discuss more chemical reactions as acid-base reactions.

Conjugate Acid-Base Pairs

\(\ce{H3O+}\) \(\ce{H2O}\)
\(\ce{H2O}\) \(\ce{OH-}\)
\(\ce{H2SO4}\) \(\ce{HSO4-}\)
\(\ce{HSO4-}\) \(\ce{SO4^2-}\)
\(\ce{NH4+}\) \(\ce{NH3}\)
\(\ce{NH3}\) \(\ce{NH2-}\)
\(\ce{H3PO4}\) \(\ce{H2PO4-}\)
\(\ce{H2PO4-}\) \(\ce{HPO4^2-}\)
\(\ce{HPO4^2-}\) \(\ce{PO4^3-}\)
\(\ce{CH3COOH}\) \(\ce{CH3COO-}\)
\(\ce{CH3NH3+}\) \(\ce{CH3NH2}\)

According to the Bronsted-Lowry theory of acids and bases, an acid is a proton donor and a base is a proton acceptor. Once an acid has given up a proton, the remaining part can be a proton acceptor, and thus a base. In this regard, an acid and a base are closely related to one another.

\(\mathrm{H^+ + {\color{Blue} Base} = {\color{Red} Conjugate\: acid\: of\: Base}^+}\)

\(\mathrm{{\color{Red} Acid} = H^+ + {\color{Blue} Conjugate\: base\: of\: Acid}^-}\)

For example:

\(\mathrm{{\color{Blue} NH_3} + H_2O \rightleftharpoons {\color{Red} NH_4^+} + OH^-}\)

\({\color{Red} \mathrm{HAc}} \rightleftharpoons \mathrm{H^+} + {\color{Blue} \mathrm{Ac^-}}\)

Thus, \({\color{Red} \ce{NH4+}}\) and \({\color{Blue} \ce{NH3}}\) are a pair of conjugate acids and bases, as are \({\color{Red} \ce{HAc}}\) and \({\color{Blue} \ce{Ac-}}\).

The table lists conjugate acid-base pairs for your reference so that you can figure out the strategy of identifying them. Furthermore, for an acid or base, you should be able to give its conjugate base or acid.

Confidence Building Questions

  1. What is the conjugate base of \(\ce{HF}\)?

Answer... F-
Consider... \(\ce{HF \rightleftharpoons H+ + F-}\)

  1. What is the conjugate base of benzoic acid, \(\ce{C6H5COOH}\)?

Answer... C6H5COO-
Consider... \(\ce{C6H5COOH \rightleftharpoons H+ + C6H5COO-}\)

  1. What is the conjugate acid of benzoate, \(\ce{C6H5COO-}\)?

Answer... C6H5COOH
Consider... Recall the previous question.

  1. What is the conjugate base of ammonium ion \(\ce{NH4+}\)?

Answer... NH3
Consider... \(\ce{NH4+ \rightleftharpoons NH3 + H+}\)

  1. What is the conjugate acid of sulfate ion \(\ce{SO4^2-}\)?

Answer... HSO4-
Consider... This is the conjugate base of \(\ce{H2SO4}\);  \(\ce{HSO4-}\) can be either an acid or a base.

  1. What is the conjugate acid of \(\ce{HSO4-}\)?

Answer... H2SO4
Consider... \(\ce{H2SO4}\) has no charge.